1. Field of Invention
The invention relates to a hydraulic control apparatus and a hydraulic control method of a vehicular automatic transmission which selectively achieve a plurality of speed change stages by combinations of engagement of a plurality of hydraulic friction engagement devices. More particularly, the invention relates to a technology that identifies a hydraulic friction engagement device having a bad engagement that has caused a failure in obtaining a speed change stage corresponding to a shift output, if such a failure occurs.
2. Description of Related Art
The hydraulic control apparatus of an automatic transmission has hydraulic friction clutches and brakes used for speed shifts, and an electronic control device that includes a microprocessor. The hydraulic actuators of the friction clutches and brakes are respectively provided with control valves. The hydraulic control apparatus of an automatic transmission further has electromagnetic changeover valves and pressure control valves for controlling the hydraulic actuators. The electromagnetic changeover valves and the pressure control valves are electrically controlled by the electronic control device, in accordance with a vehicle operation parameter, that is, one of the speed change stage of the automatic transmission, the load of the engine, and the vehicle speed. For example, Japanese Patent Application Publication No. JP-A-2002-533630 describes a hydraulic control apparatus of a vehicular automatic transmission in which when the electronic control device or the like has a failure, the electromagnetic valves and the pressure control valves are switched to a non-electric current state, and the gear is automatically changed to a low speed gear (particularly, the 3rd speed gear) if a low-speed gear has been set at the time of the failure, and the gear is automatically changed to a high-speed gear (particularly, the 5th speed gear) if a high speed gear has been set at the time of the failure.
According to the hydraulic control apparatus as described above, if a failure occurs while the vehicle is running in a low speed gear, for example, one of the 1st to 3rd speed gears, the automatic transmission is changed to the 3rd speed gear stage. If a failure occurs while the vehicle is running in a high speed gear, for example, one of the 4th to 6th speed gears, the automatic transmission is changed to the 5th speed gear stage. Therefore, the sharp deceleration at the time of occurrence of a failure can be resolved, and the vehicle can be taken off again.
However, in the aforementioned hydraulic control apparatus of a vehicular automatic transmission, when a failure occurs, the site of occurrence of the failure is not identified. Therefore, there are problems of degraded controllability during the running of the vehicle with a failure, and increased repair man-hours at service plants.